1. Field of the Invention
The present invention relates to mobile radio communication technologies, and more particularly, to a technology for reducing interference between frequencies close to each other at a cell boundary or a sector boundary of a base station having a plurality of transmission antennas in a mobile radio communication system employing the orthogonal frequency division multiple access (OFDMA) method.
2. Description of the Related Art
A mobile radio communication system is formed of a plurality of base station apparatuses and a plurality of mobile terminals (or radio terminals) communicating in radio with the plurality of base station apparatuses. The plurality of base station apparatuses are disposed separately. Each of the plurality of base station apparatuses has an area called a cell where radio communication is possible in a range where radio waves sent from the apparatus reaches. In some cases, each base station apparatus has a plurality of radio wave reaching zones called sectors made by dividing the cell when using a directional antenna. In general, the cell is divided into three sectors or six sectors. Each sector can be regarded as a cell made by dividing a space by the angle determined by the directivity of the antenna used. In the following description, cells and sectors may be collectively called cells.
The radio communication system has a mechanism for maintaining communication by successive handover among the base station apparatuses as a mobile terminal moves. To allow this handover, the cells formed by the plurality of base station apparatuses overlap. When a mobile terminal communicates in radio with a base station apparatus in an overlapped region, the communication causes interference at another base station apparatus which includes the region in its cell. Interference is a disturbance in communication performed by another mobile terminal and causes a reduction in signal quality in radio communication and a reduction in throughput.
Technologies for avoiding or controlling inter-cell interference include those disclosed in Japanese Unexamined Patent Application Publications No. 2008-61250, No. 2009-21787, and No. 2009-44397.
In standardization activities, the Third Generation Partnership Project (3GPP), which is one of standardization groups, has proposed a radio method called long term evolution (LTE), which is based on orthogonal frequency division multiplexing (OFDM). A technology for changing the antenna transmission mode of a terminal by an instruction from a base station in LTE is disclosed in 6.3.2 Radio resource control information elements, TS 36.331, 3GPP.
Another standardization group has proposed a radio method called mobile worldwide interoperability for microwave access (Mobile WiMAX), which is based on OFDM, in IEEE 802.16e. A technology called fractional frequency reuse (FFR) is described in Section 4.2 Fractional Frequency Reuse, Part I A Technical Overview Performance Evaluation, Mobile WiMAX.
FFR is also discussed in Section 20.1 Interference Mitigation Using Fractional Frequency Reuse, 08/003r6 System Description Document, IEEE 802.16m. Network multiple input multiple output (MIMO), where a plurality of base stations cooperate, is described in another section of the same document, Section 20.2.2 Multi-ABS joint Antenna Processing, 08/003r6 System Description Document, IEEE 802.16m.